Method of producing thin ferromagnetic layers of uniaxial anisotropy



United States Patent Ofifice 3,160,575 Patented Dec. 8, 1964 Recently, thin metallic ferromagnetic layers of several hundred to several thousand Angstrom units, A., socalled thin films, have gained considerable scientific interest and technical significance, particularly for use as highspeed-storage elements in the communications field. They consist of ferromagnetic metals or metal alloys, preferably composed so as to have the crystal anisotropy constant K and the magnetostriction constant approximately zero. The thin metallic layers are applied by vaporizing the respective metals or metal alloys from one or several vaporizing sources (spirals, shuttles or crucibles of high-temperature resistant material, for instance W, Ta, Al O etc.) in a high vacuum torr) onto suitable carrier substances (fine-polished glass, polished ceramic sheets, etc.), which, in some cases may be heated before they are coated. To produce a uniaxial anisotropy, a homogeneous static magnetic field of a definite magnitude is allowed to react in the direction of the supporting surface, or, the uniaxial anisotropy which is an important feature of the magnetic field may be produced in a magnetic field by a separate annealing process. Although the thin layers produced are polycrystalline, they behave like a unitary magnetic domain because of their uniaxial anisotropy. The direction of the spontaneous magnetization is parallel to the direction of the induced uniaxial anisotropy. The magnetic reversal of such a layer takes place starting at a definite magnitude of an outside magnetic field which is effective parallel to the layer through coherent rotation of the magnetization vectors to the second stable position of the magnetization direction, which is counter to the original magnetization direction by 180". The thus obtained times for magnetic reversals are of the magnitude of some 10* sec. and are smaller by two to three orders of magnitude than in the ordinarily used ferrite ring cores with rectangular hysteresis loops.

This invention describes a method of producing thin ferromagnetic layers consisting of soft magnetic ferrites which possess a uniaxial anisotropy, which is produced either during the making of the layer under the influence of a suitable magnetic'field or by subsequent annealing in a magnetic field. For this purpose the invention provides the application of the methods employed in the production of thin metallic layers by high vacuum evaporation, cathodic evaporation or the electrolytic precipitation upon such ferrites which, as to their composition, exhibit uniaxial anisotropy. After annealing in a magnetic field, certain ferrites of the sof category show the phenomenon c-f uniaxial anisotropy, with a crystal-anisotropy-constant K compensated to approximately zero by the addition of a small quantity of Co-ferrite or cobalt ferrite-forming components, and having a magnetostriction constant which, by suitably apportioned addition of Fe O likewise is brought to values near zero. Without annealing in the magnetic field these ferrites show constricted hysteresis loops, also called Perminvar loops, while under heat treatment in the longitudinal, or transverse magnetic field, they display rectangular, or Isop'erm hysteresis loops, respectively. The directional designations, longitudinal or transverse, refer to the relative position of the annealing and the measuring field to each other. According to the invention all those soft magnetic ferrites are suitable for producing thin layers of uniaxial anisotropy which have an Fe O content of more than 50 mol percent and which have a Co-oxide content of 0.1-5 percent by weight, preferably 0.3-1 percent by weight. The following examples describe the process of the invention. I

EXAMPLES 1 PRODUCTION OF A Ni-FERRITE LAYER OF UNIAXIAL ANISOTROPY A. By Oxidation 0] Metallic Layers ((2 The raw material consists of a Ni-FeCo alloy of the following range of composition:

This is deposited by evaporation from a suitable evaporating crucible, for instance, A1 0 or MgO, in a high vacuum p 10 torr) upon a suitable substrate, which may be heated, for instance, fire-polished glass, polished A1 0 or the like, to a layer thickness of several hundred or several thousand A. units. The layer thus produced is polycrystalline and is further treated, in accordance with the invention, as follows:

It is carefully oxidized at a temperature range between 600 and 1000 C. bysuitable dosage of the oxygen content of a nitrogen-oxygen mixture until the Rontgenor electron diffraction clearly discloses the spinel lattice of the ferrites. At this stage of the oxidation the oxygen feed is cut off, and during the cooling of the ferrite layer from some ten degrees above the Curie temperature down to room temperature, a static magnetic field effective in the layer plane for the production of the desired uniaxial anisotropy is applied.

The primary metal layer may also be produced, without restricting the scope of the invention, by high vacuum evaporation of the alloy components from separately located evaporation sources.

This modification of the process has the advantage that in case of diiferent evaporation temperatures of the individual alloy components, the heat may be individually adapted and the concentration adjusted, for example, by rotating sector shutters in front of the evaporation sources.

(a The primary metal layer may further be produced by cathodic sputtering and deposition in vacuum by means of oxidation-inhibiting sputtering gases, for instance, rare gases, N or also H at a pressure of 10 to 10* torr, wherein the cathode employed is a metal alloy of the above-indicated composition range, or separate cathode blocks, consisting of the individual alloy components may be employed. The latter will always be advantageous when the sputtering conditions for the individual alloy components differ Widely and require differentiated sputtering potentials. Further, in this manner the concentration of the metal layer applied through the rotating sector shutters may be more readily controlled.

(:1 In another version, the primary metal layer can be produced by the electrolytic methods ordinarily employed for electroplating, such as described, for instance, by I. W. Wolf and V. P. McConnell, Proc. Am. Electroplaters Soc. 1-4, 1956, and I. W. Wolf, H. W. Katz, A. E. Brain, Proc. 1959, Electronic Component Conf., pages 15-20.

B. By Direct Layer Formation (5 By cathodic sputtering and dep0siti0n.Again the raw material is a Ni-FeCo alloy of the composition described under (al However, the sputtering gas is now oxygen of 10 to 10* torr, which completely oxidizes the disposed metal atoms of the alloy on the way to the substrate, resulting in the formation of a ferrite layer. Suitably, in this process the substrate is heated to tem- 4 EXAMPLES 2 Further examples with respect to an alternate choice of ferrite components, see col. 1 of the following table.

The layers produced according to one of the examples peratures of about 8Q0 C. again, to produce the uni- 5 under (1) of the composition (Ni1 x yCoXFey)O Fe2O3 aXlal anlsotropyr durlng coolmg Pmcess down to are polycrystalline, show the spinel structure of the ferrites room temperature magnetlc fie 1d Parallel to the and are ferromagnetic, with the following properties: plane of at least twice the coercive force of the ferr1te is applied. To prevent further oxidation, the vacuum samfatlon magnetlzatmn l 34O0 3800 bell jar is rinsed with a neutral gas or pumped to a high cunetempemmm 540-580 Vacuum of Coerc1ve force 2) ferrite evfllloration high Without The hysteresis loop, is, (a) (measured parallel to the crucible.--The raw material consists of a Perminvar feru iaxi l i t an i o erm 100p. rite of the composition (Ni Co Fe )O.Fe O This B ferrite is produced in form of cylinders about the thick- 0 85 ness of a pencil by extrusion of the raw ferrite body BB prepared according to ceramic methods and subsequent (b) (measured perpendicularly to the direction of the sintering. Two such ferrite cylinders which are pointed uniaxial anisotropy): an isoperm loop. are brought into point contact and connected to a high The magnetic reversal by coherent rotation required voltage source. These cylinders are now heated from 20 some 10 sec. an outside source, whereupon the resistance thereof be- As heretofore described in the introduction, it is possicause of their negative resistance temperature coefiicient, ble, in principle, to produce soft magnetic ferrite layers is quickly reduced and a current begins to flow through according to the methods of the invention with the charthe cylinders, which in turn heats up the pointed ends acteristic of uniaxial anisotropy, insofar as their composiof the ferrite cylinders to the melting and evaporation tion includes a suitable Fe and Co content and actemperature. As in the case of the carbon arc evaporacordingly have the following formula tion, ferrite particles are thus thrown on a substrate M 4. 2+

e o Fe e about 20 cm. distant, and the substrate 1s coated with 1 x yc x y)OF 203 the desired ferrite layer. The uniaxial anisotropy may be Wlth the hmlts produced, in accordance with the invention, either immediately following the layer formation process by cool- 3% 111 a magnet: field effefctlve Parallel to layer The designation Me is used to indicate the following plane from a temperature slightly above the Curre tembivalent metal ions, which may be present i l or i perature down to room temperature in the hlgh vacuum, combination with each other, always, however, in such a or by a Separate annealing Process by heatmg to a manner that their total mol content does not go above perature of some ten degrees above the Curie point and 1 subsequent cooling in the magnetic field. 2 2 2 2 2 ([2 By ferrite evaporation from a crucible in high N1 Mn Mg Cu Z Cd vacuum-The raw material consists of a Perminvar-ferrite Cases in which a (Zn-Co-Fe) ferrite, or a (Cd-Co-Fe) P YQ t accofdlflg t0 Ceramic P ocesses of the com- 40 ferrite, or also a (Zn-Cd-Co-Fe) ferrite would be obposition (N1 Co Fe )O.Fe O The powder is placed tained must be excluded. in a crucible of A1 0 Mg c In gh vacuum The following table summarizes the processes which evaporation apparatus of conventlonal design and at presmay be successfully applied in accordance with the insures of 1 O torr heated by electric current and finally vention for thin ferromagnetic layers with uniaxial anisoevaporated. 0 tropy of various Perminvar ferrite systems and some of The temperatures required n this instance are between the properties of such layers.

Production of primary metal layers Directlayer HV-evaporation Properties production, cathodic H'v .Qathodic dispersion Coercive Me evaporation dispersion Orucible- Crucible Saturation force Electroltree evapevaporamagnet, parallelto Br/Bl To, ys1s oration tion B, U.A. axis, degrees Jointly Sepa- Jointly Sepa- Jointly Sepa- A/cm.

rarely rately rately 3400-3s00 2.5-8 0.85 540-580 3800-4200 1.5-0 0. 30 330-380 i 25003300 3 2-10 0. 85 280-350 3000-4500 0.0-2.5 0.85 100-540 35004000 0.2-3 0.80 100-350 1- c 1200-1600 0. 5-3.0 0.80 330 T 2500-3800 0.2 .5 0.35 100-330 $1 4 iaim-4200 0. 6-6.0 0. 80 330-5 30 MnpfMg 0 u 000-05500 0.8-8.0 0.85 280-300 *Where Ni content high, otherwise separately. (II) Not necessary if N i-content'high. Not necessary. 11 Poly-crystalline. 7 Spinel structure. 1400 and 1600" C. Thefurther treatment of the layer The layers produced as described in the invention have precipitated upon the carrler takes place in the manner the advantage over ferromagnetic layers heretofore known heretofore described under (b and used in that they are stable with respect to oxidation To obtam a layer which 1s homogeneous as to the and need not be protected by coats of SiO as was thus far proportion of its constituents, the carrier or crucible is necessary. Due to their high electric resistance the elecsuitably covered by ashutter until the complete melting trical conductors required for magnetization may be diof the contents thereof and only then is released for rectly applied to these layers according to known methods, vaporlzatlon. of printed circuits, etc.

wherein 0.001 x 0.05 and"0.05 y 0.15, to said base plate, Me++ being a bivalent metal ion selected from the group consisting of Ni; Mn++, Mg++, Cu++, mixtures thereof with each'other, and mixtures thereof with Zn and with Cd++.

and thermally treating the applied ferrite from a temperature of about 10 C. above the Curie temperature thereof down to about room temperature in a static magnetic field to produce a state of uniaxial anisotropy thereof.

2. The process as claimed in claim 1 wherein said ferromagnetic material is applied to said plate by first depositing thereon, the metallic components, Me, Co and Fe in the proportions required in said ferrite and then treating the metals to oxidize the same to said ferrite.

3. The process as claimed in claim 2 comprising applying the metallic components to the base plate by cathodically sputtering the separate metals onto said base plate.

4. The process as claimed in claim 2 comprising providing an alloy of the metal components in the proportions desired in said ferrite and cathodically sputtering the alloy onto said base plate.

5. The process as claimed in claim 1 comprising providing the metallic components of said ferrite and cathodically sputtering the metallic components onto said base plate in the presence of oxygen.

6. The process as claimed in claim 5 wherein said metallic components are provided in the form of an alloy with the metal components thereof in the proportion required in said ferrite.

7. The process as claimed in claim 1 comprising providing a preformed ferrite of said composition, evaporating said ferrite under the influence of a high vacuum and redcpositing the ferrite directly onto said base plate.

8. As a new article of manufacture a basis plate of refractory material, at least one portion thereof being coated with a layer of several hundred to several thousand Angstrom units thick of a ferrite of the formula (Me++ Co++ Fe++ O.Fe O where Me++ is selected from the group consisting of Ni++, Mn++, Mg++, Cu++, mixtures thereof with each other,

and mixtures thereof with Zn++ and with Cd++, and wherein 0.001 x 0.05 and 0.05 y 0.15.

References Cited in the file of this patent UNITED STATES PATENTS 2,906,682 Fahnoe et a1. Sept. 29, 1959 2,960,457 Kuhlman Nov. 15, 1960 2,970,112 Pierrot et al Jan. 31, 1961 3,100,194 Van der Burgt Aug. 6, 1963 OTHER REFERENCES Blois: Preparation of Thin Magnetic Films and Their Properties, Journal of Applied Physics, vol. 26, No. 8, August 1955, pp. 975980. I

Williams et al.: Magnetic Domain Patterns on Thin Films, Journal of Applied Physics, vol. 28, No. 5, May 1957, pp. 548-555. 

1. THE METHOD OF PRODUCING THIN FERROMAGNETIC LAYERS WITH UNIAXIAL ANISOTROPY COMPRISING PROVIDING A BASE PLATE APPLYING A LAYER OF A THICKNESS OF SEVERAL HUNDERED TO SEVERAL THOUSAND ANGSTROM UNITS OF A FERROMAGNETIC FERRITE MATERIAL OF THE FORMULA 